A service set identifier (SSID) is a 32-character unique identifier attached to the header of packets sent over a wireless local area network (WLAN) that functions as a password when a mobile device tries to connect to a basic service set (BSS). Each set of wireless devices communicating directly with each other is called a BSS. Several BSSs can be joined together to form one logical WLAN segment, referred to as an extended service set (ESS). A SSID is simply the 1–32 byte alphanumeric name given to an ESS.
The SSID thus differentiates one WLAN from another, so all access points (a hardware device or a computer's software that acts as a communication hub for users of a wireless device to connect to a wired LAN) and all devices attempting to connect to a given WLAN must use the same SSID for the given WLAN. A device is typically not permitted to join a BSS unless it can provide the unique SSID for the BSS. An SSID is also referred to as a network name because it in effect is a name that identifies a (wireless local area) network.
A WLAN includes one or more access points and two or more stations. For example, a WLAN (ESS) for a department of a company may have several access points and include dozens of stations, all using the same SSID. Another department of the company, located in the same building, may operate its own departmental WLAN, composed of access points and stations using a different SSID. The purpose of the SSID is to help stations in say the first department find and connect to access points for the WLAN of the first department, ignoring access points belonging to the second department.
Each access point advertises its presence several times per second by broadcasting beacon frames that carry the ESS name, i.e. the SSID. Stations can discover access points for an ESS by passively listening for beacons, or they can send probe frames to actively search for an access point with the SSID of the ESS. Once the station locates an appropriately-named access point, it can send an associate request frame containing the desired SSID. The access point replies with an associate response frame, also containing the SSID.
Wireless devices thus use the SSID to establish and maintain connectivity. As part of an association process, a wireless network interface card (NIC) for a station of a WLAN must have the same SSID as the (one or more) access point(s) for the WLAN. Traditional access points are only capable of supporting a singe SSID, but now some companies are offering enterprise-class access points that support multiple SSIDs. Multiple SSIDs allow users to access different networks through a single access point. Network managers can assign different policies and functions for each SSID, increasing the flexibility and efficiency of the network infrastructure. The use of multiple SSIDs means more flexibility when deploying a shared WLAN infrastructure. Instead of supporting only one type of application, possibly one that requires significant authentication and encryption, the WLAN can also maintain other types of applications.
In order to indicate the availability of WLAN services, and as indicated above, a WLAN access point regularly sends broadcast beacon messages containing its SSID. A single WLAN access point can broadcast one or multiple SSIDs, each in a separate beacon message.
In addition to the broadcast SSIDs, a WLAN access point may support other SSIDs not advertised in the beacon messages. There are several reasons why a WLAN access point might not broadcast some SSIDs. First, while most WLAN access points support multiple SSIDs, they can usually send beacon messages only for one of those SSIDs. Second, in case of private (e.g., corporate or residential) WLANs, sending beacon messages may make it easier for outsiders to detect the presence of a WLAN network.
The only way for a mobile WLAN terminal to find out about these additional SSIDs is to send a broadcast probe message, asking if any WLAN access point nearby supports a particular SSID. When a WLAN access point supporting that SSID receives a probe message, it replies with a probe response message.
Thus, a WLAN terminal finds out about the broadcast SSIDs from all available WLAN access points by passively receiving the broadcast information (beacon messages). But this does not guarantee that all available SSIDs are known to the WLAN terminal.
In order for a WLAN terminal to send probe messages, it needs to know the SSID values it is interested in, which are included in the WLAN terminal in what is called a preferred SSID list. A preferred SSID list can be quite long, and sending a probe message for each SSID on the list can have a significant impact on a WLAN terminal's power budget, and can also affect the WLAN terminal's time and radio capacity. It would therefore be advantageous to be able to avoid sending a probe message for each SSID on a preferred SSID list.
The above-described situation has not been addressed by the prior art, since the preferred SSID list is related to the 3GPP-WLAN interworking terminal. Current WLAN solutions (for laptops) expect the user to manually select one of the SSIDs sent in beacon messages, or to type in the SSID that should be probed. Thus, a situation where the WLAN terminal would have to probe multiple SSIDs does not arise.